Lights, front and rear, at least 2 of each - it's horrifying to realise that you've just ridden 13 miles in driving rain, and your rear light has failed.
Layers of thin merino. Keeps you warm even in the rain, and can adjust easily by adding or removing layers.
Gloves: at least 2 different weights for warmer and colder days.
A buff.
Jacket - I'm happy enough after several mis-purchases with my NightVision jacket, but I bet there are better choices.
(just a start)